The present invention relates generally to a segmented coil for a transformer coupled plasma (TCP) source. More specifically, the present invention relates to a segmented coil configuration as a source for generating a plasma which can be used for treating semiconductor wafers in low pressure processing equipment.
Plasma generation is useful in a variety of semiconductor fabrication processes, for example enhanced etching, deposition, etc. Plasmas are generally produced from a low pressure gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency (RF) electric field.
Numerous techniques have been proposed to accelerate the electrons in an RF electric field. For example, U.S. Pat. No. 4,948,458 discloses a plasma generating device in which electrons are excited in an RF field within a chamber using a single winding coil (SWC) that is parallel to the plane of a semiconductor wafer and the plasma. As shown in FIG. 1, a typical plasma generating device includes RF circuitry comprising an RF source 30 coupled via a coaxial cable 32 through an impedance matching circuit to an SWC 20. The impedance matching circuitry includes a loop 34 and a coil 36. A tuning capacitor 38 is also provided as part of the RF circuitry to adjust the circuit resonant frequency to the RF driving frequency. A process gas is introduced into a chamber 10, and an RF current produced by the RF source 30 is resonated through the SWC 20, causing a planar magnetic field. The magnetic field causes a circulating flow of electrons between the coil 20 and the wafer W which is supported by a surface 40, generating a plasma in the chamber 10.
A typical SWC is shown in detail FIG. 2. The SWC 20 comprises a singular conductive element formed into a planar spiral or a series of connected concentric rings. The SWC 20 includes an inner tap, i.e. terminal, labeled (+) and an outer tap, i.e. terminal, labeled (−) so that it can be connected to the RF circuitry. Hence, coil 20 includes an inner terminal, an outer terminal and an arcuate conductor portion having at least one turn connected between the inner and outer terminals.
When processing semiconductor wafers in plasma gas environments, it is desirable to uniformly process the entire surface of the wafer. The single winding monolithic coil, such as that described, above does not provide the same uniformity at all operating pressures. For example, at one pressure the center of the wafer may be etched at a higher rate than remaining portions of the wafer while at another pressure the center of the wafer may be etched at a lower rate than remaining portions of the wafer. Other process parameters, such as RF power, gas species and flows, may also affect the rate at which the center is etched.
Several methods have been proposed to optimize plasma uniformity. For example, U.S. Pat. No. 4,615,755 discloses a plasma etching technique wherein uniformity of the wafer temperature is achieved by He backcooling of a wafer supported on a bowed electrode. By bowing the wafer away from the lower electrode with the cooling helium, cooling performance of the wafer is sacrificed in order to achieve etch uniformity. However, variations in the thickness of the wafer result in sub-standard control of the wafer bowing, thereby reducing the etch uniformity.
Another proposed solution for optimizing plasma uniformity is to adjust the process parameters. A problem with this proposed solution is that once process parameters are adjusted to obtain uniformity, only a small parameter range is typically available for optimizing other etch responses, such as profile.
Yet another proposed solution for optimizing plasma uniformity is to modify the reactor configuration by shaping the plasma window or by adding shrouds, spacers, or focus rings. The disadvantage of this approach is that such modifications are fixed and work best in a limited parameter range.
There is thus a need for a plasma generating coil that optimizes plasma uniformity but does not limit the parameter ranges needed for optimizing other etch responses.